1. Field of the Invention
The present invention relates to a liquid crystal display device and driving method thereof. Particularly the present invention relates to a liquid crystal display device in which a Wide Viewing Angle mode and a Narrow Viewing Angle mode can be easily selected by a user, and a driving method thereof.
2. Description of the Related Art
In general, a liquid crystal display device displays image by controlling optical transmittance of liquid crystal materials. This is done by injecting liquid crystal materials between two substrates (one has common electrode and the other has pixel electrode) and applying an electric field to the liquid crystal through electrodes facing each other with the liquid crystal therebetween.
A liquid crystal display device according to the direction of the electric field applied to the liquid crystal layer, can be categorized into a vertical electric field type and a horizontal electric field type.
The vertical electric field type LCD device drives the liquid crystal layer between a pixel electrode and a common electrode through a vertical electric field (vertical to an LCD panel surface). The common electrode of the upper substrate and the pixel electrode of the lower substrate are all transparent electrodes. Thus, high aperture ratio can be easily provided However, a disadvantage of this type LCD device is that a viewing angle range is narrowed to about 90°. This results from the movement of the liquid crystal which affects the light passing through the substrate in an oblique direction as the liquid crystal moves in the vertical direction to the substrate due to vertical electric field.
The horizontal electric field type LCD device is so-called In-Plane-Switching mode (IPS) LCD device. The IPS LCD device is driven by a horizontal electric field (horizontal direction to an LCD panel surface) between the pixel electrode and the common electrode arrayed on the lower substrate. In this mode, there is hardly a movement in the vertical direction as the liquid crystal is driven in the horizontal direction mainly. Therefore, the device has an advantage of having a wide viewing angle range of about 160°. Therefore, there is little effect on the light which passes through the substrate in an oblique direction.
FIGS. 1A and 1B illustrate a simplified related art IPS mode liquid crystal display device. In particular, it illustrates that the common electrode and the pixel electrode are arrayed in the pixel area. FIGS. 1A and 1B show that the IPS mode liquid crystal display device has the common electrode, the pixel electrode, upper and lower part substrates and polarizers.
According to FIGS. 1A and 1B, the IPS mode liquid crystal display device comprises a thin film transistor substrate (lower substrate) and a color filter substrate (upper substrate) which faces each other with a liquid crystal layer 10 therebetween and a spacer which maintains a cell gap between the two substrates.
The thin film transistor substrate comprises a gate line and a data line defining a pixel unit on a lower substrate 1, a thin film transistor formed at the crossing point of the gate line and the data line, a common electrode 5 and a pixel electrode 7 forming a horizontal electric field, and an alignment layer deposited on the common electrode and a pixel electrode for the initial alignment of the liquid crystal.
The color filter substrate comprises a color filter to present colors on the upper substrate 11, a black matrix to prevent light leakage between the neighboring color filters and the alignment layer deposited on the color filter and the black matrix for the initial alignment of the liquid crystal.
A lower polarizer 3 and an upper polarizer 13 polarizing incident light from a back light unit are adhered on the outside of the upper and lower substrates 1, 11. As shown in FIG. 1, the transmitting axes (polarizing axis of the polarizer) of the lower polarizer 3 and the upper polarizer 13 are perpendicular to each other. The linearly polarized light polarized by the lower polarizer 3 is transmitted into the liquid crystal materials. If the power is off, then the liquid crystal maintains its initial state. Therefore, the phase change due to the liquid crystal does not occur, and the polarizing direction does not change and transmits is the light. The direction of the linearly polarized light is perpendicular to the polarization axis of the upper polarizer 13. Therefore the linearly polarized light cannot pass through the upper polarizer 13. In other words, it shows the NB condition (Normally Black: dark screen appears when the power is off).
When an electric field is applied between the upper and lower substrates 1, 11, a liquid crystal 10 changes its alignment state according to the supplied signal. The operation of dark and bright display modes of the liquid crystal at the IPS mode would be described in detail hereinafter.
The display of dark screen is described with reference to FIG. 1A. The light polarized through the lower polarization substrate 3 enters into liquid crystal molecules 10A. The liquid crystal molecules are arrayed in parallel in an initial alignment direction as the electric field is not formed. A long axis of the liquid crystal molecules 10A is parallel to the transmitting axis of the polarizer 3. In addition, the long axis is initially aligned by the alignment layer to be in 90° as shown in the FIG. 1A. As a result, the polarization status does not change as the phase delay does not happen even if the polarized light enters into the liquid crystal molecules 10A. The light which enters into the liquid crystal molecules 10A is blocked because it cannot pass through the upper polarizer 13 having a transmitting axis perpendicular to the polarization direction of the lower polarizer 3. Therefore the liquid crystal display device shows a dark screen.
The display of a bright screen is described with reference to FIG. 1B. An electric field is formed between the electrodes 5, 7 and the liquid crystal molecules are rotated by the electric field. As a result, the liquid molecule is twisted. In general the twisted liquid molecules 10B are twisted to be in an angle of 45° to the transmitting axis of the lower polarizer 3 on average. The light polarized through the lower polarizer 3 has a phase delay as it passes through the twisted liquid crystal molecules 10B. The phase of the light polarized by the lower polarizer 3 is delayed by λ/2 along with the twisted liquid crystal 10B. Therefore, the light axis of the light incident from the lower polarizer 3 changes to 90°. The light axis of the light which has passed through the twisted liquid crystal molecules 10B is parallel to the transmitting axis of the upper polarizer 13 and therefore passes through the upper polarizer 13. Accordingly, the liquid crystal display device shows a bright screen.
The IPS mode liquid crystal display device has a wide viewing angle compared to the vertical electric field mode liquid crystal display device. The device having a wide viewing angle has an advantage in that the viewer can see images within a wide viewing angle range. However, in some cases such as using computers for personal purpose or conducting a security-required work at banks or insurance companies, a narrow viewing angle LCD device is preferred.